This invention relates to an improved porous body fluid heater and, more particularly, to a porous ceramic body utilized as an electrical heating element to raise the temperature of a fluid passing through the pore structure of the body.
Porous bodies or structures have been employed as electrical resistance heaters for fluids, particularly gases, which pass through the pores of the body or structure. A porous heater body or structure having random profuse pores and intertwining passages therethrough provides a highly efficient means of imparting heating to a fluid passing through the body. Porous bodies have traditionally been formed of granular materials such as carbon, and filamentary materials such as a compressed or felted mass of metal coated or otherwise electrically conductive fibers. Ordinarily these porous bodies have temperature limitations when used as electrical resistance heater elements and are, or may be, excessively reactive to certain reactive fluids passing therethrough. Porous ceramics have been proposed where inertness is a criteria. Ordinarily, ceramics are electrically non-conducting and require extensive modification for use in an electrical resistance heating circuit, whereas a high temperature porous material with a positive temperature coefficient of electrical resistivity is most desired in an electrical resistance heater. Additionally, ceramic bodies are usually produced as high density low porosity structures, characteristics which are not conductive to fluid flow therein.
A porous high temperature resistant ceramic material which has a positive temperature coefficient of electrical resistivity, P.T.C., is favorably inert, and can be produced in a wide range of porosity, is a metal carbide. Metal carbides are electrically conductive composite bodies of metal carbide crystals or small particles, the porosity of which may be controlled by selection of particle size for sintering, addition of filler materials and use of metal foaming processes. Examples of such metal carbides are the refractory metal carbides of such metals as tungsten, W, zirconium, Zr, and molybdenum, Mo.
A highly desirable ceramic for this invention is one which is electrically conductive with a positive temperature coefficient of resistivity, high temperature resistant, chemically inert, and has low density and high thermal conductivity. One example of such a desirable porous ceramic material for this invention is silicon carbide, SiC, which is intrinsically electrically conducting, i.e. without reliance on added materials for electrical conductivity, and embodies the other noted attributes. Silicon carbide can be produced by fusing sand and coke at a temperature above about 4000.degree. F. to form large crystals of silicon carbide which are then crushed to provide smaller grains primarily for extensive use as an abrasive, in the range from 100-1000 mesh. However, silicon carbide finds other uses such as high temperature semiconductors and cathodes, and will withstand high temperatures to its decomposition temperature of about 4200.degree. F. Silicon carbide may be produced as self-bonded low density and high density silicon carbide foams. Low density silicon carbide foam has a density of about 17 lbs/ft..sup.3 with a 90% porosity, and high density silicon carbide foam has a density of about 33 lbs./ft..sup.3 with 80% porosity. Also, various additive metals in small particle form may be added to a mass of silicon carbide crystals to increase crystal to crystal bonding or modify the electrical characteristics of all or a part of the sintered body. A high desirable electrical P.T.C. porous silicon carbide body may be closely matched in electrical and physical characteristics not only to its function of being utilized as an electrical heater for a fluid passing therethough, but also matched to specific fluids. Silicon carbide has been found to be desirably inert to various hot chemical process fluids which are reactive to other porous body materials when rapidly heated to high temperatures while in contact with the porous body material. A preferred silicon carbide body of commensurate strength and electrical conductivity has a porosity in the range of from about 30% to about 50%.
Other metal carbide bodies of satisfactory porosity, inertness and electrical conductivity which may be gainfully employed in this invention include the refractory metal carbides including, for example, tungsten, W, titanium, Ti, and tantalum, Ta.